This invention relates to the type of steam generator having a tube sheet, a heat-exchanger tube bundle formed by a plurality of interspaced tubes having ends mounted in the tube sheet and extending therefrom and a cylindrical wall which surrounds the tube bundle. To hold the tube bundle centered inside of the cylindrical wall, one or more tube spacer grids are used each formed by a ring encircling the tube bundle inside of the cylindrical wall, the ring mounting criss-crossed bars forming openings through which the tube bundle's individual tubes extend, and the ring being connected to the inside of the cylindrical wall to in this way hold the tube bundle centered within the wall.
The above is exemplified by the typical pressurized-water reactor steam generator. In this case the cylindrical wall is formed by the shroud within the vertical steam generator housing which has its bottom end closed by the horizontal tube sheet with the tube bundle being of inverted U-shape with the bottom ends of its tube legs mounted in the tube sheet. Inlet and outlet manifolds connect the inlet and outlet legs of the tube bundle with the main coolant loop of the reactor, and the vertical housing has a feed-water inlet, its top being provided with a steam dome having a steam output outlet. The shroud encircles the tube bundle and is spaced inside of the housing to form therebetween the descent space, the feed-water rising inside of the shroud and descending via this descent space to maintain a circulation within the steam generator housing. In this case the tube bundle should be held centered within the cylindrical shroud.
The individual tubes are held in their interspaced relationship by a spacer grid formed by criss-crossed bars forming openings through which the individual tubes extend, the ends of the bars being mounted by a ring encircling the tube bundle and which is connected to the inside of the cylindrical wall surrounding the tube bundle to hold the latter centered within the cylindrical wall.
When the operating temperature of the steam generator changes, thermal expansion and contraction causes relative movement between the tube bundle and wall surrounding this tube bundle. In the case of a pressurized-water reactor steam generator typically having a vertical tube bundle surrounded by a vertical shroud with vertical dimensions in the area of 10 m, and which between cold and hot conditions involve temperature differences of around 300.degree. C., the tube bundle can receive substantial mechanical stress when the spacer grid is fixed to the shroud; and since the tube bundle tubes are of relatively small diameter as compared to their length, this has been a problem. Any steam generator having a tube bundle surrounded by a cylindrical wall with the tube bundle centered within the wall in substantially the same manner, involves the same problem.